The basic economic advantages that can be realized from automating mask alignment procedures in mask defect inspections have been projected many times in the past but technically workable solutions have not been set forth. In the prior art the positioning and inspection of such masks has generally been performed by individual operators at microscopes with the operators being trained to recognize and manually position the masks and to manually position and measure on a statistical sampling basis different types of geometrical defects in the mask. By using a statistical sampling method less than 100% inspection occurs. The inspection costs are minimized but yield is effective only to the degree of accuracy of the statistical basis used for the inspection.
Previous attempts to automate mask alignment procedures in mask defect inspections have been unsuccessful primarily because of the difficulty in avoiding erroneous defect indications caused by positional errors between the respective areas on the masks which occur due to stepping errors in the production equipment used to create the mask. Such prior inventions did not provide means for resolving this stepping error from defects or other variations in the masks, especially when multiple, complex mask geometries were to be aligned or inspected.
The present invention resolves or avoids these difficulties and is an automatic alignment and defect inspection system for semiconductor device masks. This invention relies on the measurement of either reflected or transmitted light, reflected or secondary electrons, or back-scattered currents when a controllable beam is selectively addressed onto the mask with controlled field alignment and especially on specified utilitarian aspects of the mask which can be used as the alignment target for the controlled field alignment.
The present invention can be further improved if after alignment of each area to be measured, each measurement overlaps the previous measurement.
Still further the present invention achieves a significant improvement over the prior art for it compares the measured data against the original design data thus obtaining an accuracy of approximately 100 percent with higher resolution geometries.